18 research outputs found
Frictional Behavior of Atomically Thin Sheets: Hexagonal-Shaped Graphene Islands Grown on Copper by Chemical Vapor Deposition
Single asperity friction experiments using atomic force microscopy (AFM) have been conducted on chemical vapor deposited (CVD) graphene grown on polycrystalline copper foils. Graphene substantially lowers the friction force experienced by the sliding asperity of a silicon AFM tip compared to the surrounding oxidized copper surface by a factor ranging from 1.5 to 7 over loads from the adhesive minimum up to 80 nN. No damage to the graphene was observed over this range, showing that friction force microscopy serves as a facile, high contrast probe for identifying the presence of graphene on Cu. Consistent with studies of epitaxially grown, thermally grown, and mechanically exfoliated graphene films, the friction force measured between the tip and these CVD-prepared films depends on the number of layers of graphene present on the surface and reduces friction in comparison to the substrate. Friction results on graphene indicate that the layer-dependent friction properties result from puckering of the graphene sheet around the sliding tip. Substantial hysteresis in the normal force dependence of friction is observed with repeated scanning without breaking contact with a graphene-covered region. Because of the hysteresis, friction measured on graphene changes with time and maximum applied force, unless the tip slides over the edge of the graphene island or contact with the surface is broken. These results also indicate that relatively weak binding forces exist between the copper foil and these CVD-grown graphene sheets
Photochemical Reaction in Monolayer MoS<sub>2</sub> <i>via</i> Correlated Photoluminescence, Raman Spectroscopy, and Atomic Force Microscopy
Photoluminescence
(PL) from monolayer MoS<sub>2</sub> has been
modulated using plasma treatment or thermal annealing. However, a
systematic way of understanding the underlying PL modulation mechanism
has not yet been achieved. By introducing PL and Raman spectroscopy,
we analyze that the PL modulation by laser irradiation is associated
with structural damage and associated oxygen adsorption on the sample
in ambient conditions. Three distinct behaviors were observed according
to the laser irradiation time: (i) slow photo-oxidation at the initial
stage, where the physisorption of ambient gases gradually increases
the PL intensity; (ii) fast photo-oxidation at a later stage, where
chemisorption increases the PL intensity abruptly; and (iii) photoquenching,
with complete reduction of PL intensity. The correlated confocal Raman
spectroscopy confirms that no structural deformation is involved in
slow photo-oxidation stage; however, the structural disorder is invoked
during the fast photo-oxidation stage, and severe structural degradation
is generated during the photoquenching stage. The effect of oxidation
is further verified by repeating experiments in vacuum, where the
PL intensity is simply degraded with laser irradiation in a vacuum
due to a simple structural degradation without involving oxygen functional
groups. The charge scattering by oxidation is further explained by
the emergence/disappearance of neutral excitons and multiexcitons
during each stage
Spectroscopic Visualization of Grain Boundaries of Monolayer Molybdenum Disulfide by Stacking Bilayers
Polycrystalline growth of molybdenum disulfide (MoS<sub>2</sub>) using chemical vapor deposition (CVD) methods is subject to the formation of grain boundaries (GBs), which have a large effect on the electrical and optical properties of MoS<sub>2</sub>-based optoelectronic devices. The identification of grains and GBs of CVD-grown monolayer MoS<sub>2</sub> has traditionally required atomic resolution microscopy or nonlinear optical imaging techniques. Here, we present a simple spectroscopic method for visualizing GBs of polycrystalline monolayer MoS<sub>2</sub> using stacked bilayers and mapping their indirect photoluminescence (PL) peak positions and Raman peak intensities. We were able to distinguish a GB between two MoS<sub>2</sub> grains with tilt angles as small as 6° in their grain orientations and, based on the inspection of several GBs, found a simple empirical rule to predict the location of the GBs. In addition, the large number of twist angle domains traced through our facile spectroscopic mapping technique allowed us to identify a continuous evolution of the coupled structural and optical properties of bilayer MoS<sub>2</sub> in the vicinity of the 0° and 60° commensuration angles which were explained by elastic deformation model of the MoS<sub>2</sub> membranes
Understanding Coulomb Scattering Mechanism in Monolayer MoS<sub>2</sub> Channel in the Presence of <i>h</i>‑BN Buffer Layer
As
the thickness becomes thinner, the importance of Coulomb scattering
in two-dimensional layered materials increases because of the close
proximity between channel and interfacial layer and the reduced screening
effects. The Coulomb scattering in the channel is usually obscured
mainly by the Schottky barrier at the contact in the noise measurements.
Here, we report low-temperature (<i>T</i>) noise measurements
to understand the Coulomb scattering mechanism in the MoS<sub>2</sub> channel in the presence of <i>h</i>-BN buffer layer on
the silicon dioxide (SiO<sub>2</sub>) insulating layer. One essential
measure in the noise analysis is the Coulomb scattering parameter
(α<sub>SC</sub>) which is different for channel materials and
electron excess doping concentrations. This was extracted exclusively
from a 4-probe method by eliminating the Schottky contact effect.
We found that the presence of <i>h</i>-BN on SiO<sub>2</sub> provides the suppression of α<sub>SC</sub> twice, the reduction
of interfacial traps density by 100 times, and the lowered Schottky
barrier noise by 50 times compared to those on SiO<sub>2</sub> at <i>T</i> = 25 K. These improvements enable us to successfully identify
the main noise source in the channel, which is the trapping–detrapping
process at gate dielectrics rather than the charged impurities localized
at the channel, as confirmed by fitting the noise features to the
carrier number and correlated mobility fluctuation model. Further,
the reduction in contact noise at low temperature in our system is
attributed to inhomogeneous distributed Schottky barrier height distribution
in the metal–MoS<sub>2</sub> contact region
Electron Excess Doping and Effective Schottky Barrier Reduction on the MoS<sub>2</sub>/<i>h</i>‑BN Heterostructure
Layered hexagonal boron nitride (<i>h</i>-BN) thin film
is a dielectric that surpasses carrier mobility by reducing charge
scattering with silicon oxide in diverse electronics formed with graphene
and transition metal dichalcogenides. However, the <i>h</i>-BN effect on electron doping concentration and Schottky barrier
is little known. Here, we report that use of <i>h</i>-BN
thin film as a substrate for monolayer MoS<sub>2</sub> can induce
∼6.5 × 10<sup>11</sup> cm<sup>–2</sup> electron
doping at room temperature which was determined using theoretical
flat band model and interface trap density. The saturated excess electron
concentration of MoS<sub>2</sub> on <i>h</i>-BN was found
to be ∼5 × 10<sup>13</sup> cm<sup>–2</sup> at high
temperature and was significantly reduced at low temperature. Further,
the inserted <i>h</i>-BN enables us to reduce the Coulombic
charge scattering in MoS<sub>2</sub>/<i>h</i>-BN and lower
the effective Schottky barrier height by a factor of 3, which gives
rise to four times enhanced the field-effect carrier mobility and
an emergence of metal–insulator transition at a much lower
charge density of ∼1.0 × 10<sup>12</sup> cm<sup>–2</sup> (<i>T</i> = 25 K). The reduced effective Schottky barrier
height in MoS<sub>2</sub>/<i>h</i>-BN is attributed to the
decreased effective work function of MoS<sub>2</sub> arisen from <i>h</i>-BN induced <i>n</i>-doping and the reduced effective
metal work function due to dipole moments originated from fixed charges
in SiO<sub>2</sub>
Observation of Charge Transfer in Heterostructures Composed of MoSe<sub>2</sub> Quantum Dots and a Monolayer of MoS<sub>2</sub> or WSe<sub>2</sub>
Monolayer transition metal dichalcogenides
(TMDs) are atomically
thin semiconductor films that are ideal platforms for the study and
engineering of quantum heterostructures for optoelectronic applications.
We present a simple method for the fabrication of TMD heterostructures
containing MoSe<sub>2</sub> quantum dots (QDs) and a MoS<sub>2</sub> or WSe<sub>2</sub> monolayer. The strong modification of photoluminescence
and Raman spectra that includes the quenching of MoSe<sub>2</sub> QDs
and the varied spectral weights of trions for the MoS<sub>2</sub> and
WSe<sub>2</sub> monolayers were observed, suggesting the charge transfer
occurring in these TMD heterostructures. Such optically active heterostructures,
which can be conveniently fabricated by dispersing TMD QDs onto TMD
monolayers, are likely to have various nanophotonic applications because
of their versatile and controllable properties
Fano Resonance and Spectrally Modified Photoluminescence Enhancement in Monolayer MoS<sub>2</sub> Integrated with Plasmonic Nanoantenna Array
The
manipulation of light-matter interactions in two-dimensional atomically
thin crystals is critical for obtaining new optoelectronic functionalities
in these strongly confined materials. Here, by integrating chemically
grown monolayers of MoS<sub>2</sub> with a silver-bowtie nanoantenna
array supporting narrow surface-lattice plasmonic resonances, a unique
two-dimensional optical system has been achieved. The enhanced exciton–plasmon
coupling enables profound changes in the emission and excitation processes
leading to spectrally tunable, large photoluminescence enhancement
as well as surface-enhanced Raman scattering at room temperature.
Furthermore, due to the decreased damping of MoS<sub>2</sub> excitons
interacting with the plasmonic resonances of the bowtie array at low
temperatures stronger exciton–plasmon coupling is achieved
resulting in a Fano line shape in the reflection spectrum. The Fano
line shape, which is due to the interference between the pathways
involving the excitation of the exciton and plasmon, can be tuned
by altering the coupling strengths between the two systems via changing
the design of the bowties lattice. The ability to manipulate the optical
properties of two-dimensional systems with tunable plasmonic resonators
offers a new platform for the design of novel optical devices with
precisely tailored responses
Junction-Structure-Dependent Schottky Barrier Inhomogeneity and Device Ideality of Monolayer MoS<sub>2</sub> Field-Effect Transistors
Although
monolayer transition metal dichalcogenides (TMDs) exhibit superior
optical and electrical characteristics, their use in digital switching
devices is limited by incomplete understanding of the metal contact.
Comparative studies of Au top and edge contacts with monolayer MoS<sub>2</sub> reveal a temperature-dependent ideality factor and Schottky
barrier height (SBH). The latter originates from inhomogeneities in
MoS<sub>2</sub> caused by defects, charge puddles, and grain boundaries,
which cause local variation in the work function at Au–MoS<sub>2</sub> junctions and thus different activation temperatures for
thermionic emission. However, the effect of inhomogeneities due to
impurities on the SBH varies with the junction structure. The weak
Au–MoS<sub>2</sub> interaction in the top contact, which yields
a higher SBH and ideality factor, is more affected by inhomogeneities
than the strong interaction in the edge contact. Observed differences
in the SBH and ideality factor in different junction structures clarify
how the SBH and inhomogeneities can be controlled in devices containing
TMD materials
Simultaneous Hosting of Positive and Negative Trions and the Enhanced Direct Band Emission in MoSe<sub>2</sub>/MoS<sub>2</sub> Heterostacked Multilayers
Heterostacking
of layered transition-metal dichalcogenide (LTMD)
monolayers (1Ls) offers a convenient way of designing two-dimensional
exciton systems. Here we demonstrate the simultaneous hosting of positive
trions and negative trions in heterobilayers made by vertically stacking
1L MoSe<sub>2</sub> and 1L MoS<sub>2</sub>. The charge transfer occurring
between the 1Ls of MoSe<sub>2</sub> and MoS<sub>2</sub> converted
the polarity of trions in 1L MoSe<sub>2</sub> from negative to positive,
resulting in the presence of positive trions in the 1L MoSe<sub>2</sub> and negative trions in the 1L MoS<sub>2</sub> of the same heterostacked
bilayer. Significantly enhanced MoSe<sub>2</sub> photoluminescence
(PL) in the heterostacked bilayers compared to the PL of 1L MoSe<sub>2</sub> alone suggests that, unlike other previously reported heterostacked
bilayers, direct band transition of 1L MoSe<sub>2</sub> in heterobilayer
was enhanced after the vertical heterostacking. Moreover, by inserting
hexagonal BN monolayers between 1L MoSe<sub>2</sub> and 1L MoS<sub>2</sub>, we were able to adjust the charge transfer to maximize the
MoSe<sub>2</sub> PL of the heteromultilayers and have achieved a 9-fold
increase of the PL emission. The enhanced optical properties of our
heterostacked LTMDs suggest the exciting possibility of designing
LTMD structures that exploit the superior optical properties of 1L
LTMDs
Simple Chemical Treatment to n‑Dope Transition-Metal Dichalcogenides and Enhance the Optical and Electrical Characteristics
The
optical and electrical properties of monolayer transition-metal
dichalcogenides (1L-TMDs) are critically influenced by two dimensionally
confined exciton complexes. Although extensive studies on controlling
the optical properties of 1L-TMDs through external doping or defect
engineering have been carried out, the effects of excess charges,
defects, and the populations of exciton complexes on the light emission
of 1L-TMDs are not yet fully understood. Here, we present a simple
chemical treatment method for n-dope 1L-TMDs, which also enhances
their optical and electrical properties. We show that dipping 1Ls
of MoS<sub>2</sub>, WS<sub>2</sub>, and WSe<sub>2</sub>, whether exfoliated
or grown by chemical vapor deposition, into methanol for several hours
can increase the electron density and also can reduce the defects,
resulting in the enhancement of their photoluminescence, light absorption,
and the carrier mobility. This methanol treatment was effective for
both n- and p-type 1L-TMDs, suggesting that the surface restructuring
around structural defects by methanol is responsible for the enhancement
of optical and electrical characteristics. Our results have revealed
a simple process for external doping that can enhance both the optical
and electrical properties of 1L-TMDs and help us understand how the
exciton emission in 1L-TMDs can be modulated by chemical treatments